1) The message and design of the landing page. 2) The message and media used to drive traffic to the page.
Low quality on either will result in low conversion (5% of traffic).
A highly targeted campaign that points out a pain point and a landing page that is well designed and speaks to that pain converts at a far higher rate. I now frequently see 15-20%. I should note that those rates are for registered users, not for other metrics farther down the line (some sort of interaction for example).
Two pages that convert registered users at 20%: http://www.styckyd.com/sell_race_cars, http://www.styckyd.com/sell_modified_cars
Facebook ads have been the most successful generator of quality traffic for us so far.
Everything is hand coded and designed by me from scratch. I use Rails/HAML to make things a little easier.
Quora has some interesting posts on the topic (http://www.quora.com/Landing-Pages). For reference, our landing page is http://beta.opperator.com
Ambient conversion - below 1% you're doing something wrong, above 1% you're doing something right.
Something people really want - below 10% you're doing something wrong, above you're doing something right.
This is a gross over-simplification, but these conversion rates can be applied to just about anything on the web (ad click thrus, free account signups, purchases, even % of people who will go from one page on your site to the next).
Regular (more random) traffic at about 20%.
It depends a lot on where the traffic is coming from (a targeted ad, article, a search, ...)